This invention relates generally to electrooptical displays of the electrochromic type wherein the electrochromic material undergoes reversible color changes upon the application of an electric field. More particularly, the invention discloses an electrochromic display which includes an improved rear electrode with special electroreactive layer. The invention further relates to a process for making the improved electrochromic display of the present invention.
U.S. Pat. No. 3,840,287 to H. Witzke et al. which issued Oct. 8, 1974 describes a typical electrochromic cell having a symmetrical type structure. The patent discloses a color promoting agent such as a palladium layer having hydrogen associated therewith located between a first and second layer of electrochromic material and respective electrode layers which are on the outside of said electrochromic layers. The Witzke patent, in effect, discloses a cell arrangement represented as follows: SnO.sub.2 /WO.sub.3 /Color Assisting Agent/WO.sub.3 /SnO.sub.2 ; wherein SnO.sub.2 represents a conductive tin oxide layer and WO.sub.3 represents an electrochromic tungsten oxide layer. The feature of the Witzke cell is the unbalanced arrangement wherein one of the electrochromic layers is initially in its colored state so that current can readily flow through this layer. Thus, one of the electrochromic layers is colored after the cell is completed by exposure to ultraviolet radiation or another suitable process.
U.S. Pat. No. 3,827,784 to R. D. Giglia et al. which issued Aug. 6, 1974 is also of interest in this area and discloses an electrooptical data display formed by a sandwich arrangement of the imaging area and the counter electrode area with a suitable ion conducting layer therebetween. The counter electrode comprises a bonded graphite tungsten oxide mixture which improves the electrochromic reversibility, speed, and cell life over prolonged coloration and erase cycles.
Other background art of general interest in the area of electrochromic displays include U.S. Pat. No. 3,521,941 to Deb et al., issued July 28, 1970 which discloses various electrochromic materials and U.S. Pat. Nos 3,283,656 to Jones, issued Nov. 8, 1966, and 3,704,057 to Beagle which disclose various displays using electrochromic materials. Also of interest is the pending application of M. Leibowitz, Ser. No. 540,481 filed Jan. 13, 1975 which is assigned to the assignee of record of this application.
A serious problem in existing electrochromic displays is the difficulty in achieving a hermetic seal to prevent air from leaking into the display. On the prior art one of the electrochromic layers serves as a source of hydrogen ions which travel from rear to front electrode by ionic conduction through the electrolyte to provide color in the front layers of electrochromic material. If the seal leaks air into the cell, the hydrogen will combine with oxygen in the air and become depleted. It is necessary to provide sufficient hydrogen to provide the color changes for display purposes and to respond to an applied voltage. Attempts have been made to solve this problem by generating hydrogen during cell usage but such an arrangement requires a rather high voltage. Another solution has been to impart molybdenum into the tungsten oxide to make it photosensitive and then expose the material to light to produce a blue hydrogen layer. This arrangement, however, generally requires three separate voltages, one to turn on the display, a second to turn off the display and a third to maintain it off.
A further important advantage of this invention is that the display operates at a lower voltage than prior art displays since it is not necessary to generate hydrogen from the electrolyte. Having a lower operating voltage also prolongs cell life since operating at higher voltages over a prolonged period tends to destroy the display.
Accordingly, an object of this invention is to provide a new and improved electrochromic display having a treated electroreactive layer which improves the operating and life characteristics of the display.
Another object of this invention is to provide a new and improved electroreactive layer for an electrochromic display which can also serve as rear electrode.
Another object of this invention is to provide a process for producing a new and improved electroreactive electrode for an electrochromic display.
A more specific object of this invention is to provide a new and improved electrochromic display of a sandwich type construction and a method for making said display wherein a rear electroreactive layer is oxidized and heat treated in situ to provide a display having a lower operating voltage and increased display life.
Another object is to provide an improved display, wherein the conventional rear electrode coated with electrochromic material may be alternatively integrated into a single layer construction.